This paper provides a comprehensive demographic analysis of the family structure experiences of children in the U.S. Childbearing and transitions among co-residential union states defined by single, cohabiting, and married are analyzed jointly. A novel contribution is to distinguish men by their relationship to children: biological father or stepfather. This distinction is rarely made when analyzing union formation, but it is critical for understanding the family structure experiences of children. The analysis uses data from the 1979 cohort of the National Longitudinal Survey of Youth (NLSY79). The results are used to address the following issues: (1) What fraction of their childhood do children spend with the biological father, stepfathers, and no father? (2) How do these fractions vary by the mother's marital status at the time of the child's birth and at the time of the child's conception? (3) How do the family structure experiences of the children of white, black, and Hispanic mothers differ, and what are the proximate demographic determinants of these differences? A key finding is that children of black mothers spend on average only 34.1% of their childhood living with the biological father and mother, compared to 72.8% for whites and 64.1% for Hispanics. The two most important proximate demographic determinants of this large racial gap are the much higher propensity of black women to conceive children outside of a union, and the lower rate of shotgun unions for blacks compared to whites and Hispanics. Another notable finding is that cohabitation plays a negligible role in the family structure experiences of children of white and Hispanic mothers, and even for children of black mothers accounts for only one fifth of time spent living with both biological parents. Finally, we find that children of black, Hispanic, and white mothers spend similar proportions of their lives with stepfathers present, but this similarity masks a much higher stepfather turnover rate among blacks, who are more likely than the other groups to experience a larger number of shorter spells with different stepfathers.